Our own technological development is so rapid that we must accept the fact of imminent developments which are beyond our present understanding... It is therefore certain that a society only a few hundred years more advanced than our own could, if they thought it expedient, exterminate terrestrial life without effort...

It is useless to think of any form of defence against any action by superior intelligeneces... it would be just as futile as the occupants of an antheap declaring war against a bulldozer.

Cade (1966), p. 220

Page 223 includes this drawing of self-reproducing machines.